Thermochemistry Jayme Bartz Group Members: Tommy, Kari, and Erica Section 226 Kasun Imaduwage XJayme BartzIntroduction: Hess’s Law states that the total enthalpy change in a chemical reaction is independent of the path used to get between the initial and final states. It is known as the principle of conservation of energy. In this experiment, a calorimeter was used to prove Hess’s Law. Calorimetry is the science of measuring the heat of chemical reactions, this is done by using a calorimeter which distinguishes between heat and temperature. The purpose of the experiment was to measure the temperature changes in chemical reactions to be plugged into the equation, Heat released or absorbed = (mass)x(specific heat)x(change in temperature) If this reaction is releases energy through heat or light, then it is exothermic and the answer is negative. If it absorbs energy, then it is endothermic and has a positive answer. Specific heat is the amount of heat per unit mass required to raise the temperature of a substance by one degree Celsius. The purpose of the second part of the experiment was to use the same elements and procedure, but change the states and concentrations of each. Experimental: Procedures for this experiment were gotten from the course website. For part 1 of the experiment, a calorimeter was constructed using two Styrofoam cups and a temperature probe which was connected to the computer. The file 18 Hess’s Law was opened for recording. All reagents were weighed and water and HCl were placed in the calorimeter. The initial temperature data was collected. NaOH was then added to the solution and was the solution was mixed until the maximum temperature was reached. The final temperature was taken and used to find the change in temperature. Results were plugged in the change in heat equation in joules. The volume of each reagent, system mass, moles of HCl and NaOH, and stoichiometric ratio were also found. A drop of indicator was then added to the beaker and agitated. Our data was then compared with the other groups. For the second part of the experiment, the procedure was repeated with the volumes of Group 3 that gave the correct stoichiometric ratio of 1:1. The heat of the reaction was determined. The experiment was then again repeated with solid NaOH instead of liquid. Another variation was then performed with solid NaOH and water without HCl. The final variation was repeated using acetic acid instead of HCl. The change in heat was determined for each variation. Results: Heat absorbed/released = (mass)x(specific heat)x(change in temperature) was used to find that the change in heat for Team 1 was -833.21 Joules. The specific heat used was 4.184 j/g*C. The change in temperature is determined by subtracting the initial temperature from the final. For Part 2, the heat was found by using the same equation to get Joules, then divided by moles to get the change in heat. The chemical equations for each reaction are as follows. 2A) NaOH(aq) + HCL(aq) – NaOH(aq) + HCl(aq) 2B) NaOH(s) + HCl(aq) – NaOh(aq) + HCl(aq) 2C) NaOH(s) + HCl(l) – NaOH(aq) + HCl(aq)Part 1 Parameters Team 1 Team 2 Team 3 Team 4 Team 5 NaOH volume 24.0 mL 25.0 mL 25.0 mL 25.0 mL 25.0 mL HCl volume 15.0 mL 20.0 mL 25.0 mL 30.0 mL 35.0 mL Water volume 20.0 mL 15.0 mL 10.0 mL 5.0 mL 0 mL System mass 63.22 g 60.87 g 59.41 g 61.31 g 66.71 g Moles NaOH .025 .025 .025 .025 .025 Moles HCl .015 .020 .025 .030 .035 NaOH:HCl 1.67:1 1.23:1 1:1 1:1.2 1:1.4 Temp change 3.15 C 5.3 C 5.15 C 5.73 C 5.05 C Heat change -833.21 J -1095.12 J 1280 J 1.470 J 1305.2 J Indicator observation Dark pink pink Bright pink No change No change Part 2 Parameters 2A 2B 2C NaOH volume/mass 25. 0 mL 25.0 mL 25.0 mL HCl/acetic acid volume 25.0 mL 25.0 mL 25.0 mL Water volume 10.0 mL 10.0 mL 10.0 mL System mass 60.78 g 35.61 g 10.34 g Change in temp 5.67 C 15.64 C 19.59 C Heat -1430 J -2330 J -847.51 J Change in heat -57200 J/mol -93200 J/mol -33900 J/mol Discussion: Our strategy for this experiment was to use the calorimeter to find the change in temperature so that we could use other givens and factors to plug into the equation. We tried to eliminate some percentage of error by running the calorimeter the same way each time and by stopping the timer each time when the maximum temperature dropped by .01. Some other sources of error could have come from our measurements of volume and mass and in our overall calculations. The potential impact of this error could completely throw off our stoichiometric ratio and make the whole experiment invalid. In any experiment error can have an extremely negative impact. Our goals were met in finding the change in heat for every variation of the experiment and in Part 1 our data was comparable to that of the other groups. The exothermic nature of the process was evident in the negative heat findings. The increases in temperature are also evidence of the system releasing energy in the form of heat. The enthalpy values for the solid NaOH processes were higher than the values for the aqueous NaOH which is evident in comparing part 2A and 2B. Hess’s Law is proved through these reactions, showing that all of the energy from the reactants is not lost if less product is produced, sometimes is can be converted into light, or in this case, heat. The same goes for products with more energy than their reactants because they can absorb energy throughout the reaction. With our results, we expected to get figures close to the other groups in Part 1 even with all of the variations in reagent volume. We also expected to get astoichiometric ratio of 1:1. While our results were comparable to the other groups, our ratio was way off at 1.67:1. We also expected our changes in heat to be different for each variation of Part 2 and they were. The impact of the information obtained from Part 1 was the volumes necessary to have a correct stoichiometric ratio. Another impact was how the change in heat between the groups could be explained by the direct relationship between the different system masses and changes in temperature for each group. Conclusion: The overall goal of our experiment was to use the calorimeter and heat absorbed/released equation to determine the change in heat of different combinations of NaOH, HCl, and water. The heat change of the reaction for Part 1 was -833.21 J. The heat changes in part 2 A, B, and C were -57200 j/mol, -93200 j/mol, and -33900 j/mol. The relationship between